The invention relates to capillary columns used to analyze chemical substances. More specifically, the invention is directed to a new method for creating a fluid tight seal between a capillary column and a connector.
Chromatographic apparatus used for both gas and liquid chromatography typically employ capillary columns to provide control passageways for substances to be analyzed. Areas of analytical application for capillary columns include gas chromatography, liquid microbore chromatography, capillary electrophoresis, and supercritical fluid chromatography. In most analytical applications today, glass, metal or flexible fused silica capillary columns are used. Occasionally, polymeric capillaries are also used. Frequently, it is necessary to join two pieces of capillary columns together in order to repair a broken column, to optimize a chemical separation by joining dissimilar columns, to extend the column length, or to add retention gaps or guard columns. In most analytical applications the column ends must also be connected to a sample injector and a detector.
Connectors are known in the art for receiving a fluid stream in a first fluid-bearing conduit and then delivering the received fluid stream to a second fluid-bearing conduit. In many cases, the fluid connection is obtained by manual manipulation of separate components that comprise the connector, such as by alignment and compression of a ferrule onto a column that is fitted to a receiving device.
The requirements placed upon a practical capillary connector for general use in chromatography applications are demanding. The connector must be able to withstand regular contact with chemically reactive substances and organic solvents. It must remain leak-free when operated at internal pressures ranging from zero (absolute) to several thousand pounds per square inch and over temperature cycles from sub-ambient to several hundred degrees Celsius. The thermal mass must be small and the thermal conductivity high to maintain thermal equilibrium between the column and its immediate surroundings. The dead volume in the joint must be as close to zero as possible
Many connectors have been devised to address the need for providing a fluid tight coupling between column ends and connectors. Some connectors employ a ferrule with a conical frustrum exterior and a longitudinal bore. The column end is inserted through the bore of the ferrule and then the column-ferrule assembly is inserted into the interior of the connector. The interior of the connector is shaped to receive the ferrule. Pressure is then applied to the ferrule via a threaded fastener creating a fluid tight seal. Other connectors known as press-fit connectors consist of a hollow glass elongated tube having a conical configuration. Press-fit connectors have a tapered internal bore narrowing from the end to the central portion. In use a column end is inserted into the open end of the connector and moved into the press-fit position. Press-fit glass tubes have also been used with a ferrule and compression fitting at each end to seal against the capillary column. These methods are also used to connect the ends of two columns together with a fluid tight seal.
Known connectors provide low dead volume and chemically inert connections but suffer from several disadvantages. The ferrule and compression style connectors require several moving parts and require careful assembly. Many require separate elastomeric seals to ensure fluid-tight connection. The components used to make the seal must be chemically inert to the substances used in the analytical chromatographic process, and must exhibit good temperature stability. These components increase the cost and the complexity of the fluid seal and the connector. All of these connections show increased leakage at high temperatures, increased leakage after thermal cycling, and a sensitivity to tensions/torques applied to the fluid sealing location. The drawn conical connector of the press-fit variety has been reported to suffer from inconsistent fluid seal, particularly with modern high temperature fused silica capillary columns. At elevated temperature the fluid seal has been reported to leak and come apart.
Method and apparatus for establishing a fluid seal between capillary columns and connectors with a tapered conical internal bore is described. These methods and apparatus can be used when attaching the end of a column to an injectorport or a detector. The methods and apparatus can also be used when joining two capillary columns together end to end utilizing a union connector. In addition to establishing fluid tight seals for capillary tubing, these methods and apparatus can be used on a variety of tubular objects such as on larger bore tubing such as microbore columns, and megabore columns.
The improved fluid tight seal is generally accomplished by the following steps: 1) an auxiliary length of tubing is closely fit over the outside diameter of the capillary column so that a short length of the capillary column remains exposed; 2) the capillary column is press-fit into a connector with an internal conical taper shaped bore creating a fluid seal, and; 3) the auxiliary tubing is then moved into the connector and press-fit into the conical taper section of the connector creating a second fluid seal.
One of the advantages of these methods and apparatus is that it simplifies the installation of a column into a connector. The assembly of the auxiliary length of tubing is very simple and no additional screws, ferrules or tools are required to create a fluid tight seal. The outside diameter of the auxiliary tubing need not be bonded or glued into the internal bore of the connector nor does the outside diameter of the capillary column need to be glued or bonded to the inside of the auxiliary tubing. The capillary column and auxiliary tubing are press-fit into place to create a reliable fluid seal.
When assembled, the auxiliary tubing provides an extra seal that provides several operational benefits. The increase in the fluid sealing surface area increases the stability of the fluid seal during mechanical vibration. This in turn increases the tensile force required to compromise the fluid seal which dramatically reduces the leak rate of the fluid seal. Additionally, the auxiliary tubing adds essentially no physical and thermal mass, is reliable, and is inexpensive.